Zinc refining process



June 5, 1934. C Q VMAlER ZINC REFINING PROCESS Filed Dec. 26. y19751 2Sheets-Sheet l- All/wein@ Patented June 5, 1934 UNITED STATES PATENTOFFICE 3 Claims.

My invention relates in general tothe reduciion of zinc ores. It relatesmore in particular to improved means and the process for reducing zincoxide to metallic zinc, and is a con- 5 tinuation in part of my priorapplication, Serial Number 513,458, flied February 4, 1931.

In the known art, the reduction of zinc oxide yto metallic zinc isaccomplished by the addition to the zinc oxide of some reducing agent ascarbon,

l0 a hydrocarbon or other organic compound which at elevated temperaturecombines with the oxygen of the' zinc oxide to form zinc vapor andvarious proportions of water, carbon monoxide and carbon dioxide. Theseproportionsare limited by the respective chemical equilibria involved,and are determined by chemical reaction rates in approaching theseequilibria. The zinc vapor is then condensed from this mixture bycooling. By this process as now practiced, it is not possible to obtainzinc entirely in a coherent form, but some blue powder is always formeddue to reversal at condensation temperatures of the above mentionedequilibria. This finely divided zinc and zinc-oxide mixture or bluepowder is well known to owe its origin to mechanical aswell as chemicalfactors, (l) the rapid condensation of the zinc from the vapor to thesolid or spheroidal state and (2) the reaction of the carbon dioxide,water or other oxygen compound formed by the reduction reactions athigher temperatures with ihe zinc vapor or finely divided solid orspheroidal zinc metal particles when the temperature is lowered from thereduction temperature, to form zinc oxide which coats the zincparticles. and prevents coalescence. .It will be seen from thisdiscussion that if condensation takes place slowly the amount of powderdue to factor one will be decreased but that formed by factor two willbev increased so that the best that can be hoped for 40 is a compromisewith the formation of considerable blue powder.

Other methods known to the prior art require the mixture of gases fromthe reduction of zinc oxide as above described to be passed overincandescent coke to remove the oxidizing gases before condensing thezinc. This method, however, requires the use of expensive solid fuel'and requires two steps, the second of which must be carried out at atemperature considerably above 1000 degrees centigrade which Oilersdifficulties of operation and produces less pure zinc than processescarried out at lower temperatures.

In still another method known in the prior art reducing gases such ascarbon monoxide y and hydrogen are added to the condensing chamber.

This involves an additional step in the preparation of the reducing gasand cannot substantially reduce the amount of oxidizing gas presentwithout too much dilution of the zinc vapor which causes difficulty incondensation. Carbon monoxide and hydrogen in some instances have beenused directly as reducing agents, but, such gases can reduce onlyinsignificant amounts of zinc at temperatures below 1000 C. and thecarbon dioxide or watervapor formed in the reaction readily reoxidizesthe metallic zinc when the mixed gases are cooled in the condenser toseparate liquid zinc. In this process excessive amounts of blue powderinvariably are obtained.

The principal object of my invention is the provision of an improvedprocess for producing metallic zinc.

Another object is to reduce zinc oxide by a process in which nore-oxidizing vapor such as carbon dioxide or steam can be formed.

Another object is to reduce zinc oxide at lower temperatures than havebeen used successfully heretofore.

Another object is to reduce zinc oxide to metallic zinc with theproduction of a minimum amount of blue powder.

Another object is to reduce zinc by a process which can be carried outcontinuously and economically.

Other vobjects and features of the invention will be apparent from thefollowing detailed description taken with the accompanying drawings.

In the accompanying drawings I have illustrated in diagrammatic form twomodifications of.

apparatus suitable for use in the practising of my process. In thisshowing,

Figure 1 illustrates a suitable form of apparatus to be used when thepretreatment process hereinafter'described is not employed in thepractice of the invention, and

Figure 2 illustrates a modified form of apparatus for use when thepretreatment process hereinafter described is employed in conjunctionwith my process.

In general, my invention comprises the reduction of zinc oxide tometallic zinc by means of methane as a reducing agent. the source of themethane being preferably a natural gas which contains methane in amountsgreater than approximately 80%. Other sources of methane can, however,be employed.

In carrying out my process, I control all the factors so that the' zincoxide is reduced by the methane substantially according to the reactionThis reaction may be carried out in any suitable type of retort heatedin any suitable manner to which the zinc oxide is supplied in relativelyfinely divided form. The gaseous products of thereaction are carriedfrom the retort into'a condenser where the zinc vapor is condensed 'andrun off into suitable molds for further disposition in the usual way.

Iam aware that the use 0f methane and other As previously disclosed,when such side reactions occur, it is impossible to prevent theformation of considerable amounts of blue powder, so much so.

in fact that the methane reduction of zinc oxide heretofore has beenimpracticable.

I have discovered, by the application of thermodynamic chemical methodsto the equilibria involved in the reduction process, that by controllingcertain factors including especially temperature and pressure, thedisposition of the zinc oxide with respect to the methane and theabsenceof catalytic iron, in the condensing range, that I can substantiallyinhibit side reactions and the products of the reaction will besubstantially zinc, carbon monoxide and hydrogen, with the formation ofsubstantially no carbon dioxide or water vapor, with a concomitantdecrease in the amount of blue powder formed. One of the most importantcharacteristics of my invention is the discovery that methane will notdecompose in the presence of zinc oxide. under the conditions hereindescribed (but under other conditions methane is incapable of practicaluse for zinc reduction due to excessive decomposition into carbon andhydrogen). In other words, the rate of decomposition of methane inintimate contact with zinc oxide, within certain temperature ranges, issmall as compared with the rate of reaction with zincoxide. By makinguse of this discovery, and controlling the disposition of the methanewith respect to the zinc oxide so that there is always an intimatecontact between the two, the methane will be available in the form ofCI-I4 for direct reaction with the zinc oxide. This, with a propercontrol of temperature, substantially completely avoids a possibility ofside reactions ofthe character hereinabove referred to.

As to temperature, I have found that a temperature range ofapproximately 800 to 1000" centigrade produces the best results, withthe very best temperature range between 925 to 990 centigrade. Thefollowing analysis will illustrate the results obtained in the preferredtemperature range.

centrigrade) CH4 C O K C Or Hz H2 0 Percent Perce Percent Percent 3 25.0 0. 5 5l. 2 0. 7 29. 2 0. 4 50. 2 0. 6 32. 5 0.8 66. 7 0. 9

Above this temperature range `I have found that the carbon dioxidecontent increases and below it the methane decomposes with thedeposition of carbon with consequent loss of efficiency.

By accomplishing the reduction within the= above range and maintainingthe methane throughout the heating range in intimate contact with thezinc oxide, I have secured excellentresults in producing metallic zincwith .u

minimum amount of blue powder, substantially less than is customarilyformed in the most satisfactory present commercial processes. The bestresults are obtained by a proper control of the condensing step of theprocess, particularly avoiding the presence of catalytic iron within thecondensing range, and cooling at a controlled rate which is not toorapid, this being possible as hereinabove explained due to the fact thatthe gases entering the condenser consist substantially entirely of zinc,carbon monoxide and hydrogen. In actual practice, good results attendthe practice of impinging the vapor streamy directly upon the liquidzinc surface.

It is evident that the desirable reaction occurring between zinc oxideand methane is reversible but at the desirable temperatures set out therate of the reducing reaction is so very much greater than the rate ofspeed of the reverse reaction that very high eciency results and theyield of zinc vapor is relatively very large. It is, of course, truealso that zinc oxide and carbon monoxide will react together inaccordance with the following reaction This reaction is undesirable asthe presence of relatively large amounts of CO2 in the condensing rangewill result in the formation of blue powder. This reactionalso isreversible, however, and, at the temperatures which I select, chemicalequilibria in this reaction are such as to prevent the formation ofharmful amounts of carbon-dioxide. I have found that when the carbondioxide content is kept below approximately 1% the rate of reoxidationreaction is so low that zinc vapor may But, at the temperatures which Iemploy, only very slight traces of Water vapor can be formed accordingto the equilibria involved, and slight traces of water vapor are withouteffect in the condensing range, as in the case of traces of carbondioxide.

I shall now refer to the drawings in which I illustrate one type ofapparatus with which my process may be carried out. I employ a retort l0suitably supported within a furnace 11 into which retort the ore isadapted to be inserted through an ore delivery pipe 12.. A gas deliverypipe 13 is provided adjacent the ore delivery pipe. The retort restsupon a condenser 14 and between .the retort. proper and the condenser Isupport in any suitable manner pieces of refractory material 16. Nearthe bottom of the retort proper and above the pieces of refractorymaterial I insert an ore discharge pipe 17 through which the residue ofthe ore, after the reduction of the zinc oxide, is passed. 'Ihe gases,including the products of the reaction, pass through the refractorymaterial 16 (which material holds back the ore) down into the condenser,its path through the condenser being relatively long due to the presenceof horizontal baffles 18 and vertical bafes 19'. The gases are cooled intheir path through the condenser and portions of the metallic zinc,becoming liquid, flow to the bottom of they condenser and out through atap '21. The gases, however, impinge directly onto the liquid metal-`lic zinc in the bottom of the condenser, andthe be used to pre-treatthe zinc ore, or can be used in other processes for example to removesulphur from certain ores. It will be seen that with this form ofapparatus the ore and gas may be fed continuously and concurrently orthe retort may be charged with the ore and then reduced in a batch. Iprefer, however, the continuous treatment for several reasons whichreadily suggest themselves, but principally because the intimate contactis more easily maintained -with the continuous treatment.-

One. of the advantages of the concurrent downward movement of the ,oreand gas is that the intimate contact between the gas and ore ismaintained. I have found that if this intimatecontact is maintained, ametal retort can be used with satisfaction (the relatively lowtemperature also makes this possible) since whatever'cata-4 gas, I havefound that the reduction is ineifective. Below 1000 C. substantiallyonly carbon is formed by thermal decomposition of the hydrocarbon gas,and, at these temperatures, the carbon so formed will not subsequentlyreact with zine oxide to reduce the same. formed would disappear at hightemperatures by reaction with the zinc oxide in this way, but in doingso would produce reoxidizing carbon dioxide and the effect of myinvention thus could not be obtained.

Methane gas is unstable at temperatures from 8001000 C., but iseffective for. my purpose as controlled according to my teachings. Iexplain my results by the following hypothesis which seems to t all ofmy findings. If attention is focused on an individual molecule of thisgas, it is found that a certain critical time period after itstemperature is raised (which in general is a major fraction of a second)is necessary to permit the reaction of decomposition to occur. If,however, this individual particle may be caused to come into contactwith a molecule of zinc oxide before the critical time period haselapsed, the faster reaction of methane with. zinc oxide takes place tothe exclusion of thermal decomposition. I accomplish this conjunction ofmethane and zinc oxide in less than the critical time for thermaldecomposition by maintaining it in intimate contact with the zinc oxide,preferably passing the gas downwardly through a nely granular bed of orecontaining no large voids. Thus, if using briquetted charges, I partlyll the'interstitial spaces with granular material passing throughscreens 8 meshes per inch, but remain- The carbon so ing'upon screens 40or 60 meshesv per inch.. In thisway, the molecules of gas are neverabsent from ore particles for a great enough time to allow thermaldecomposition.

The downward flow of gas makes the process relatively easilycontrollable since I have found that upward flowing gas tends to formchannels in the ore charge, producing the voids which must be eliminatedto prevent carbon formation.

The condenser is preferably made of a suitable refractory materialsubstantially devoid of catalytic iron in its constitution. Many of theordinary refractory materials of commerce are satisfactory for thepurpose. The material indicated at 16 is, for example, broken pieces ofrefractory material. By saying that zinc oxide has an anti-catalyticeffect, I mean that al'- though methane is unstable at temperaturesbetween 800 and 1000 centigrade, in the presence of zinc oxide and withsufficiently intimate contact, the reduction reaction may be made tooccur preferentially as indicated above.- For this reason, care should-be taken to avoid raising the temperature of the methane to thereaction temperature in the absence of the anti-catalytic zinc oxideeither before it enters the retort or even after entrance, and it shouldenter the retort at a point where the temperature is below 800 degreesfor best results, although some slight preheating of the methane maytake place without serious results; If care is not taken to maintain themethane in intimate contact with the zinc oxide, the carbon formed bythe breaking down of the methane will be deposited in such a way as tosubstantially choke up the retort and prevent the proper movement of thegas therethrough. I found that once carbon starts to form it has atendency to form relatively quickly but it will not initially begin itoform if an ample supply of zinc oxide is present in substantially allportions of the retort in a condition to contact intimately with themethane.

Other ways of maintaining this intimate contact may, of course, beutilized in place of the down flow principle employed in the apparatusdescribed. the simplest means of obtaining the intimate contact and hasbeen effective in producing very satisfactory results.

When I speak of the reduction of zinc oxide. it

is understood that this term zinc oxide is employed in the usual mannercustomary in the industry. It includes roasted blend which has beensubstantially freed from sulphur or any nat- This is probably, however,one of ural or processed ore consisting essentially of zinc oxide.

One very satisfactoryand novel way of practicing my invention is toemploy the exit gases of the zinc oxide reduction process as they leavelthe retort for the pretreatment of the ore, as for -which mingles withthe effluent gases of the retort and, coming in contact with liquid zincin the condenser, recombines 'to-form a fouled condensate containinginfusible zinc sulphide. The m lcerial se formed is analogousto bluepowder.

To avoid this trouble, I have found that I can remove the harmfulsulphur impurities by pretreatment of the ore at temperatures slightlylower than 800" C., using the exit gases from the retort, aftercondensation of the zinc therefrom. 'Ihis pretreatment may beaccomplished in another i'etort similar to the one used for reduction,but without the condenser, which is replaced by a settling chamber ordust separator to collect the impurities driven off, and operated attemperatures approximately from about 600 to 800 C. vUnder thesecircumstances, practically no zinc is reduced. Some other substances maybe affected by such pretreatment such as cadmium, arsenic (if present),and some other volatile impurities.

I regard the pretreatment of ore by exit gases or other reducing gasesof similar composition as an essential p art of my process andconstituting a part of the invention whereby blue powder formation isminimized., and entirely fusible and coherent substantially puremetallic zinc produced.

The o re is preferably fed to the retort at substantially thetemperature required to obtain the desired reaction described. Inactually carrying out the process, the ore after roasting can be feddirectly to the pre-treating retort and then to the reducing retort andthis can be accomplished so that it will be at a temperature onlyslightly below the reduction temperature when delivered to the reductionretort. If the pre-treatment step with the exit gas is not employed, anyother suit-- able means may be employed for assuring that lthe ore ispreheated before delivery to the reduction retort. The process can becarried on by supplying substantially cold` ore to the reduction retort.and depend on the heat supplied to the retort to raise it to thereduction temperature, but this is not the preferred practice.

The reducing apparatus should be regulated to have a pressure slightlyabove atmospheric pressure to prevent the entrance of air at any pointand it may be necessary under certain circumstances to provide a smallcounter-current of the exit gas through the' orc discharge pipe toprevent diffusion of zinc into such pipe.

In Figure 2 of the drawings, I have shown a modified form of apparatusfor use when the pretreatment process described above is employed inconjunction with my process. In this form of apparatus the constructionis similar to that' shown in Figure 1 with the exception that apretreatment retort 30 is arranged above the retort 10 and is separatedtherefrom by a partition 31 in which a bell hopper 32 is arrangedwhereby pretreated ore may be delivered `from the retort 30 to theretort l0 by operation of the lever 33. As` will be apparent from Figure2, in the modied form of apparatus shown the ore inlet pipe 12 isadapted to deliver ore into the retort 30 instead of directly into theretort 10, as in Figure 1. The gas outlet pipe 22, provided with a valve34, is connected to the inlet side of a gas pump or fan 35 which isadapted to force the exit gases from the condenser through the pipe 3 6to the gas inlet pipe 37 and into the pretreatment retort 30.

Preferably at the lower end of the retort 30 and opposite the inlet pipe37 is arranged a gas outlet pipe 38. As shown, the pipe 3.6 is alsoconnected with the pipe 39, having a valve 40, which connects with a gasinlet pipe 41 arranged in the ore discharge pipe 17, whereby a smallcountercurrent of exit gas is provided in the ore discharge pipe 17 toprevent the diffusion of zinc into such pipe.

Those skilled in the art will understand that the explanation of themanner in which the proccss is carried out and in which the apparatus isconstructed and operated is subject to considerable variation andmodification in accordance with specific conditions.' I do not restrictmyself, therefore, in any sense by the details disclosed but theinvention is limited only by the scope of the appended claims.

What I claim as new and desire to protect by United States LettersPatent is:

1. A process for reducing zinc oxide in ore to metallic zinc, whichcomprises continuously feeding zinc ore into the top of a verticallydisposed retort, heating the ore to a temperature of between 800o C. and1000 C., continuously supplying methane to the top of the retort andpassing the methane vertically downwardly while maintaining intimatecontact between the methane and zinc oxide in the heating range, wherebythe methane and zinc oxide react to produce gaseous reaction products,including principally Zinc vapor, carbon monoxide and hydrogen, passingthe gaseous products into a condenser, condensing the zinc vapor andcontinuously removing residue ore material from the bottom of theretort. f

2. A process for reducing zinc oxide to metallic Zinc, which comprisescontinuously feeding the zinc oxide in relatively ilnely divided forminto one end of a retort, heating the zinc oxide to a temperaturebetween 800 C. and-1000 C., continuously passing methane into the sameend of the retort as that into which the Zinc oxide is delivered,whereby a concurrent and confluent movement of the zinc oxide andmethane takes place through the retort, maintaining the zinc oxide andmethane in intimate contact with each other in the heating range,whereby the methane is maintained stable with the exception of areaction with zinc oxide toproduce gaseous reaction products, includingprincipally zinc vapor, carbon monoxide and hydrogen, passing thegaseous reaction products into a condenser to condense the zinc vaporand continuously removing solid residue material from the other end ofthe retort.

3. A process for reducing zinc oxide in ore to metallic zinc, whichcomprises continuously feeding zinc ore into the top of a verticallydisposed retort, heating the ore continuously to a temperature between800 C. and 1000 C., continuously supplying methane to the top of theretort and passing the methane vertically downwardly while maintainingintimate contact between the methane and zinc oxide in the heatingrange, whereby the methane and zinc oxide react to produce gaseousreaction products, including principally zinc vapor, carbon monoxide andhydrogen, passing the gaseous reduction products into a condensersubstantially free of catalytic iron, condensing the zinc vapor, andcontinuously removing residue ore vmaterial fro-m the bottom of theretort.

CHARLES G. MAIER.

